(1) Field of the Invention
The present invention relates to plasma sprayed mullite coatings and to an improved method for plasma spraying mullite coatings onto silicon based ceramic materials. More particularly, the invention relates to an improved method for plasma spraying predominantly crystalline, crack-free, mullite coatings onto silicon based ceramics and composites without the need for preheating the substrate above the crystallization temperature of mullite.
(2) Description of the Prior Art
Silicon based ceramics such as silicon carbide and silicon nitride as well as composites of these materials have attracted considerable attention in recent years due to their high temperature capability and/or thermal stability in oxidizing environments. Such materials have found wide spread interest for the fabrication of high temperature structures such as gas turbine components, combustor components, compressor linings, nozzles and heat exchanger equipment.
It has been found desirable, if not necessary, in many applications to apply a protective coating to the silicon based ceramics and composites in order to limit thermal gradients and thereby reduce thermally induced mechanical stresses within the substrate. Coatings have also been applied to these ceramic materials to provide environmental protection or to bond other environmentally protective coatings, such as those which are zirconia based, to the substrates. Ceramic coatings have also been provided in the form of abradables, for example, to provide gas path sealing for rotating turbine components and the like.
It is known that mullite (3Al2O3.2SiO2) is an excellent thermal barrier coating (TBC) for silicon based ceramics due to its low thermal conductivity. Mullite is stable in oxidizing atmospheres and also has a low thermal coefficient of expansion which nearly matches that of silicon carbide, for example. However, if mullite is thermally sprayed as a primarily amorphous coating, subsequent exposure to temperatures above about 1800xc2x0 F. (about 980xc2x0 C.) causes re-crystallization and sintering. This can result in shrinkage leading to severe surface cracking and eventually loss of the coating.
U.S. Pat. No. 5,391,404 issued to Kang N. Lee et al. on Feb. 21, 1995, discloses a method for plasma spraying adherent mullite coatings onto silicon based ceramic materials wherein a crystalline mullite powder is flame-sprayed onto a substrate inside a furnace which is heated to elevated temperatures for the spraying process. At temperatures above 1800xc2x0 F. (about 980xc2x0 C.) mullite can crystallize in the high temperature environment and the coating is observed to be free of any amorphous material after it is removed from the furnace.
One problem with this approach, however, has been that the coating must be applied at these high temperatures inside a heating furnace. This requirement limits the size and complexity of the substrate or part that can be coated using conventional thermal spray equipment. There is, therefore, a need in the industry for an improved method of plasma spraying mullite coatings onto silicon based ceramics and composites which does not require heating in a furnace.
Another problem with the approach taken by Lee et al. is that it involves depositing an amorphous aluminosilicate material onto the substrate and then recrystallizing this material in-situ within the furnace during spraying. This can result in volume shrinkage and associated stress in the coating, which can limit the coating life.
The present invention contemplates and demonstrates an improved method for plasma spraying a mullite coating onto the surface of a silicon based ceramic or composite which does not require preheating above the crystallization temperature of mullite. The invention also contemplates and demonstrates a method for plasma spraying a coating which comprises at least about 85% by volume of crystalline mullite and about 0.5% to about 15% by volume amorphous material.
The method of the invention is carried out by plasma spraying mullite powder(s) onto the surface of the substrate to be coated using a commercially available thermal spray plasma torch. During the plasma spraying operation, the nozzle outlet of the spray gun is maintained at a predetermined distance from the surface of the substrate and the substrate and nozzle outlet are moved relative to one another. The distance at which the nozzle outlet is spaced from the substrate, or the so called xe2x80x9cstand-offxe2x80x9d distance, is chosen to minimize the time the mullite powder(s) are subjected to the plasma flame. The speed at which the substrate and nozzle outlet move relative to one another is preferably selected to limit the residence time of the flame against the substrate surface and also to control the deposition rate. The feed rate of the powder(s) passing through the spray gun is also closely controlled in order to restrict the amount of plasma energy absorbed by the powder(s).
The powders used in the method of the invention are free-flowing, non-agglomerated, fused and crushed mullite powders of relatively large or coarse size, having an average particle size of about 62 microns, and a range of about 16 to 176 microns. It is believed that the coarse powders remain relatively solid when exposed to the plasma flame except for the outer surface portions of the powders which dissociate and/or soften.
It is possible according to the invention to plasma spray a mullite coating onto the surface of a silicon based ceramic or composite wherein the coating, as applied and without heating to the crystallization point of mullite, is at least 85% crystalline mullite and about 0.5% to 15% by volume total of amorphous material and dissociated phases of mullite. This is accomplished by closely controlling the spray parameters which affect the absorption of the plasma energy by the mullite powder, including the type of feedstock and its particle size, stand-off distance, powder feed rate, and the plasma parameters such as gas composition and power.
The coating of the invention as applied comprises particles of crystalline mullite in a matrix composed of amorphous material and dissociated phases of mullite. The crystalline mullite particles constitute at least about 85% by volume of the coating while the matrix constitutes about 0.5% to about 15% by volume of the coating.